Radiation oncology patients receive highly complicated treatments such as intensity modulated radiation therapy (IMRT), volumetric modulated arc therapy, stereotactic radiosurgery and stereotactic body radiation therapy using clinical linear accelerators. Currently, the quality assurance (QA) of a radiation treatment is don a priori to measure cumulative doses, which is not sensitive to temporal beam modulation. Therefore, the actual quality and safety of the delivered treatment is unknown and patient mistreatment cannot be identified or corrected. We hypothesize that if in vivo real time on line transmission detectors for external beam radiotherapy are developed, radiotherapy accidents will be prevented to achieve considerable improvement in treatment accuracy, verification, recording, and efficacy. This proposal addresses the urgent need for in vivo real time transmission detectors to provide radiation oncology clinics a novel treatment verification and control paradigm. These online, all time detectors will increase patient safety during treatment by improving external beam delivery accuracy, providing real time treatment verification, real time detection of departures from planned treatments, automatic beam halting and warnings if errors are detected, automatic QA procedures and standardization, and true determination of the spatial and temporal fluence delivered to the patient by each IMRT beam. The impact of this enhancement will improve external beam radiotherapy safety and treatment accuracy, which will improve the treatment outcome for 50% of cancer patients. In our clinic alone, this number is estimated to be 270 patients per day. In a consortium between Washington University in Saint Louis and Best Medical Int., we will develop an in vivo real time transmission detector for external beam radiotherapy using highly integrated plastic encapsulated radiation hard scintillating fibers and monolithic high gain photosensors. The high throughput electronics of the detector are constructed using parallel data conversion and parallel data acquisition integrated circuits. This fast and parallel electronic interface is capable of detecting treatment errors during delivery in real time and accordingly correct or suspend a treatment if patient health is compromised. The commercially viable detector prototype will be validated using external beam QA and anthropomorphic phantoms, delivering treatment plans for head and neck and lung cancer and formulating patient QA protocols based on real time treatment verification of these two cancer sites. The proposed imaging dosimeter can be applied to real time verification of other cancer malignancies and treatments, but in this project we will concentrate on these two sites as models and validation for the implementation of this technology to enhance the safety and therapeutic efficacy of external beam radiotherapy in current clinical practice. The low cost and autonomous capability of the proposed technology makes its implementation simple and adaptable for large and small clinical operations.